1. Field of the Invention
The present invention relates to a pneumatic tire, and more particularly to a pneumatic tire which uses a specific rubber composition in the tread and exhibits excellent performance by simultaneously achieving improvement in both controllability on wet roads (wet-road property) and chipping resistance of the tread (blockwise tread rubber dropout). The present invention also relates to a pneumatic tire which has a tread structure composed of a cap layer and a base layer and shows a suppressed decrease in the wet-road property and improved quality to vibration in a car ride (ride quality) throughout the entire period of tire use.
2. Description of the Related Art
It has heretofore been known that simultaneous improvement in abrasion resistance and the wet-road property is enabled by using a high-structure carbon black in a rubber composition for a tire tread.
The market requires a further increase in abrasion resistance. It is difficult, however, to achieve a further increase in abrasion resistance through the conventional method of using a specific type of carbon black because an increase in the amount of carbon black decreases breaking energy, which is defined as the integral of a stress-strain curb up to the elongation where the sample breaks, in later periods of tire use. Moreover, this method has another problem in that the wet-road property is weakened because of the hardening of rubber composition in later periods of tire use, and this method has been found to be insufficient for maintaining properties during the entire period of tire use. It is proposed in Japanese Patent Application Laid-Open (JP-A) No. 58-87138 that the appearance in the final period of tire use can be improved by suppressing the aging of tread rubber composition and sidewall rubber composition by using a rubber composition comprising a combination of a specific antioxidant and a specific vulcanization accelerator. However, nothing is mentioned about improvement of chipping resistance and the wet-road property in later periods of tire use. In JP-A No. 56-139542, a specific vulcanization accelerator is proposed and improvement of the workability of a rubber composition by using this vulcanization accelerator is described. However, a tire using this rubber composition has not been examined.
On the other hand, the effective vulcanization system is known as a method of suppressing the hardening of 9 rubber composition. The effect of suppressing the hardening of rubber composition is insufficient, however, and the breaking energy is decreased. Therefore, this method is not preferable with respect to tire durability.
Technologies for improving the balance between the wet-road property and rolling resistance by adding silica into a rubber composition for a tire tread are disclosed in JP-A Nos. 3-252431 and 5-271477. However, it is found by the studies conducted by the present inventors that the chipping resistance of a tire, particularly in the final period of tire use, is decreased by using silica. Methods known for improving chipping resistance include decreasing the tensile stress and the gripping property of a rubber composition. These methods cannot actually be applied, however, because the decrease in the tensile stress causes a decrease in controllability. The chipping of a tread, i.e., blockwise tread rubber dropout, in later periods of tire use is caused by a decrease in the breaking energy of the tread rubber composition. It was found by an analysis of the tread rubber composition after the tire has been used conducted by the present inventors that the hardening phenomenon takes place by heat aging of a tread rubber composition, and the breaking energy is decreased because of the phenomenon. It was also found that the main cause for the hardening phenomenon is additional cross-linking by sulfur and vulcanization accelerators remaining in the rubber composition. This hardening phenomenon causes the additional problem that the wet-road property is weakened in later periods of tire use. Therefore, maintaining properties during the entire period of tire use is another subject for improvement.
SBR having a high content of styrene, and/or silica are used for a tread rubber composition which contacts a road surface in order to improve the wet-road property of a pneumatic tire. The tread rubber composition is softened to improve ride quality. In accordance with these methods, the wet-road property and ride quality in the initial period of tire use can be improved, although, these properties are weakened in the intermediate to, final periods of tire use.
For overcoming this problem, in a method disclosed in JP-A No. 59-70205, for example, different rubber compositions are used for the cap layer and the base layer of a tread to maintain the properties by exposure of the base layer as abrasion of the tread proceeds. However, this method has a problem that the base layer has been exposed to heat and deformation within the tread and the base layer rubber composition has already aged by the time the base layer is exposed to the surface of the tread, so that the desired properties cannot be exhibited.
A method disclosed in JP-A No. 58-87138 is known to improve the appearance in the final period of tire use by suppressing the aging of rubber composition in tread and sidewalls, which are exposed to the surface. Nothing is mentioned, however, about the effect of a base rubber used at the inside of a tread. Neither, the wet-road property nor ride quality is mentioned or suggested.